In non-linear optical, or wave mixing processes, outputs are produced at sum, difference or harmonic frequencies of the input(s). Second order non-linear optics, or three wave mixing involves combining two inputs to produce one output at one of the combined frequencies. The use of second order non-linear optical surface spectroscopy to examine physical properties of a material surface is known. However, practical constraints on such known methods have impeded progress on the material evaluation and characterization beneath the surface of a material under inspection.
The Terahertz (hereinafter “THz”) range refers to electromagnetic waves with frequencies between 100 GHz and 10 THz, or wavelengths between 3 mm and 30 μm, existing in the radiation spectrum between the microwave and infrared region. Terahertz waves are known to pass through a variety of amorphous and opaque substances. In addition, many biomolecules, proteins, explosives or narcotics also feature characteristic absorption lines, so-called spectral “fingerprints”, at frequencies between 0.3 and 3 THz. The two main advantages of THz radiation are thus the penetration of conventionally opaque materials on one hand, and a high chemical selectivity on the other hand. Terahertz imaging is therefore thought to be a non-destructive technique for interrogating dielectric (non-conducting) materials. The use of terahertz waves for non-destructive evaluation of materials enables inspection of multi-layered structures and can therefore identify abnormalities from foreign material inclusions (contamination), delamination, heat damage, etc.
The spectroscopic frequency band of a 0.1 to 10 THz is not easily accessible. Electronic sources like Gunn or Schottky diodes with subsequent frequency multipliers, provide high output levels (mW range), up to some 100 GHz, yet become inefficient in the sub-millimeter range. Direct optical sources, like quantum cascade lasers, are usually limited to frequencies greater than 5 THz, even when operated at cryogenic temperatures.
Optoelectronic THz generation is an expression for indirect methods, where near-infrared laser light illuminates a metal-semiconductor-metal structure, generating a photocurrent that becomes the source of a THz wave. Both pulsed and continuous-wave (CW) techniques have been realized, and both have their advantages and limitations. Pulsed THz radiation offers a higher bandwidth (typically from about 0.1 to about 10 THz) and permits very fast measurements (a spectrum can be acquired within milliseconds). On the other hand, the frequency resolution is limited to several GHz. Vice versa, a CW system features a somewhat lower bandwidth (typically from about 0.1 to about 2 THz) and requires longer measurement times (acquiring a spectrum takes several minutes), yet the frequency can be controlled with extreme precision (down to single MHz).
Composite materials such as fiberglass, Kevlar and carbon fiber are increasingly being used as structural components in aircraft because of their high strength to weight ratios, improved performance, reduced corrosion, etc., compared with other known structural materials. However, composites can be weakened by various defects and stress during their life cycle. Further, routine maintenance of composites requires complicated inspection and repair techniques.
Terahertz radiation offers a non-invasive, non-contract, non-ionizing method of assessing composite part condition. However, THz sources have generally been difficult to produce. While there has been recent development using quantum cascade lasers, such devices remain largely in the developmental stage, and are intrinsically low-power devices. This limits their application to selected industrial manufacturing environments that require ease of operation and speed of data acquisition. In addition, use of THz surface of a substrate surface using presently available technology would still lack the degree of surface specificity required for interrogating composite and other opaque surfaces.
The use of composite materials in modern manufacturing scenarios requires the existence of diagnostics that can reliably interrogate surface and subsurface composite characteristics. Such interrogation developments were not required for interrogating previous metallic-based manufacturing, since such systems were developed based on several centuries of metal manufacturing experience. While x-ray technologies were adequate to perform subsurface measurements with metal substrates, concerns regarding health and safety of personnel have properly precluded their use and adaptation in all but the most carefully controlled environments, such as medical facilities.